Re: [CAD_CAM_EDM_DRO] Testing a 10HP Spindle with a 2HP VFD...Hmmm

Steven Ciciora wrote:

>I'm jealous! All I can say is I'm sure it's not recommended, but I run my
>4 HP Bridgeport series II spindle motor with a 2 HP VFD, no problems
>yet.
>

Yes, with a good VFD, the margins are enough to tolerate a 2:1 change
without
much trouble. But, not only is this a 10 Hp spindle, it is a 400 Hz
motor as well.
That means the inductance is 10 x 400/60 = 67 times less! That is
absolutely
not going to work, I can assure you, without series inductors between
the motor
and the VFD.

You mention braking resistors, that's another area. That high speed
spindle will
have enormous energy at 24,000 RPM. It will take a huge braking resistor to
absorb it. When I run my stock Bridgeport the braking resistors never get
warm. When I run my tiny air bearing spindle at 24,000 RPM and shut it
down,
the braking resistor gets warm after the first stop. I actually
overheated the
braking resistors once with a program that was starting and stopping
many times.
You don't have to buy the fantastically overpriced VFD braking
resistors, unless
you need UL compliance. I used 100 W vitreous enamel wirewound resistors,
that I have a small stock of, to get the right current draw at 400 V to
absorb
to motor's energy. If you know where to look, these are available on the
surplus market.

Jon